Oxide thin film transistor and method of fabricating the same

ABSTRACT

An oxide thin film transistor (TFT) and its fabrication method are disclosed. In a TFT of a bottom gate structure using amorphous zinc oxide (ZnO)-based semiconductor as an active layer, source and drain electrodes are formed, on which the active layer made of oxide semiconductor is formed to thus prevent degeneration of the oxide semiconductor in etching the source and drain electrodes.

The present patent document is a divisional of U.S. patent applicationSer. No. 12/548,908, filed Aug. 27, 2009, which claims priority toKorean Patent Application No. 10-2008-0098819 filed in Korea on Oct. 8,2008 and Korean Patent Application No. 10-2008-0099814 filed in Korea onOct. 8, 2008, which is hereby incorporated by reference.

BACKGROUND

1. Field of the Invention

The present disclosure relates to an oxide thin film transistor and itsfabrication method, and more particularly, to an oxide thin filmtransistor having a bottom gate structure using amorphous zincoxide-based semiconductor as an active layer and its fabrication method.

2. Discussion of the Related Art

As consumer interest in information displays is growing and the demandfor portable (mobile) information devices is increasing, research andcommercialization of light and thin flat panel displays (“FPD”), whichsubstitute cathode ray tubes (CRTs), the conventional display devices,has increased. Among FPDs, the liquid crystal display (“LCD”) is adevice for displaying images by using optical anisotropy of liquidcrystal. LCD devices exhibit excellent resolution, color display andpicture quality, so they are commonly used for notebook computers ordesktop monitors, and the like.

The LCD includes a color filter substrate, an array substrate and aliquid crystal layer formed between the color filter substrate and thearray substrate.

An active matrix (AM) driving method commonly used for the LCD is amethod in which liquid crystal molecules in a pixel part are driven byusing amorphous silicon thin film transistors (a-Si TFTs) as switchingelements.

The structure of a general LCD will now be described in detail withreference to FIG. 1.

FIG. 1 is an exploded perspective view showing a general LCD device.

As shown in FIG. 1, the LCD includes a color filter substrate 5, anarray substrate 10 and a liquid crystal layer 30 formed between thecolor filter substrate 5 and the array substrate 10.

The color filter substrate 5 includes a color filter (C) including aplurality of sub-color filters 7 that implement red, green and bluecolors, a black matrix 6 for dividing the sub-color filters 7 andblocking light transmission through the liquid crystal layer 30, and atransparent common electrode 8 for applying voltage to the liquidcrystal layer 30.

The array substrate 10 includes gate lines 16 and data lines 17 whichare arranged vertically and horizontally to define a plurality of pixelareas (P), TFTs (T), switching elements, formed at respective crossingsof the gate lines 16 and the data lines 17, and pixel electrodes 18formed on the pixel areas (P).

The color filter substrate 5 and the array substrate 10 are attached ina facing manner by a sealant (not shown) formed at an edge of an imagedisplay region to form a liquid crystal panel, and the attachment of thecolor filter substrates 5 and the array substrate 10 is made by anattachment key formed on the color filter substrate 5 or the arraysubstrate 10.

The above-described LCD is a display device receiving much attention sofar with its advantages of being light and consuming a small amount ofpower. However, it is a light receiving device, not a light emittingdevice, and has a technical limitation with respect to brightness, acontrast ratio, a viewing angle, and the like. Thus, development of anew display device that can overcome such shortcomings is activelyongoing.

An organic light emitting diode (OLED), one of new flat panel displaydevices, is self-emissive, having good viewing angle and contrast ratiocompared with the LCD. Because it does not need a backlight, it can beformed to be lighter and thinner, and is advantageous in terms of powerconsumption. Also, the OLED is driven at a low DC voltage and has a fastresponse speed. In particular, the OLED is advantageous in terms offabrication costs.

Research for a large-scale organic light emitting display is activelyongoing, and development of a transistor achieving a stable operationand durability by securing constant current characteristics as a drivingtransistor of the OLED is required.

The amorphous silicon thin film transistor used for the above-describedLCD may be fabricated in a low temperature process, but it mobility isvery small and does not satisfy constant current bias conditions.Meanwhile, a polycrystalline silicon thin film transistor has highmobility and satisfies the constant current bias conditions, but it isdifficult to secure uniform characteristics, so it is difficult toincrease in area and a high temperature process is required.

Thus, an oxide semiconductor thin film transistor including an activelayer as oxide semiconductor has been developed, but application of theoxide semiconductor to the thin film transistor of the conventionalbottom gate structure causes degeneration of the oxide semiconductorduring an etching process of source and drain electrodes.

FIG. 2 is a sectional view schematically showing the structure of ageneral oxide thin film transistor.

As illustrated, a gate electrode 21 and a gate insulating layer 15 areformed on the oxide thin film transistor of the bottom gate structure,and an active layer 24 formed of oxide semiconductor is formed on thegate insulating layer 15.

Thereafter, source and drain electrodes 22 and 23 are formed on theactive layer 24, and at this time, in the process of depositing andetching the source and drain electrodes 22 and 23, the lower activelayer 24 (in particular, a portion ‘A’) is possibly damaged to bedegenerated, deteriorating the reliability of the device.

Namely, when the source and drain electrodes are formed according to awet etching, the active layer is lost or damaged due to the physicalproperties of the oxide semiconductor which is weak to an etchant. Also,even when the source and drain electrodes are formed according to a dryetching, back sputtering or oxygen deficiency of the oxide semiconductorcauses the active layer to be degenerated.

Although not shown, a protection layer (passivation layer) made ofsilicon oxide film is formed by using a plasma enhanced chemical vapordeposition (PECVD) equipment, and in this case, in the oxidesemiconductor constituting the active layer, hydrogen atoms serve as acarrier within the semiconductor thin film by a reaction with H2 gaswhile the PECVD silicon oxide film is deposited, making the oxidesemiconductor changed to a conductor.

Thus, in place of the single protection layer of the silicon oxide film,a dual structure in which an etch stopper that restrains a reaction withH2 gas is additionally formed on the active layer is employed, which,however, has a complicated process and incurs high cost. In addition,when the protection layer of the dual structure is formed by adjusting arate of flow of the H2 gas of the reaction gas, particles are generateddue to precipitation of silicon atoms, disadvantageously narrowing aprocess window.

BRIEF SUMMARY

An oxide thin film transistor (TFT) includes: a gate electrode form on asubstrate; a gate insulating layer formed on the gate electrode; sourceand drain electrodes formed on the gate insulating layer and having amulti-layer structure of two or more layers; and an active layer formedon the source and drain electrodes and formed of amorphous zincoxide-based semiconductor, wherein a metal layer such asindium-tin-oxide, molybdenum, and the like, having good ohmic-contactcharacteristics with titanium and a titanium alloy having good bondingforce with oxygen or the oxide-based semiconductor is formed at anuppermost portion of the source and drain electrodes.

A method for fabricating an oxide thin film transistor, including:forming a gate electrode on a substrate; forming a gate insulating layeron the substrate; forming source and drain electrodes on the gateinsulating layer; and forming an active layer made of amorphous zincoxide-based semiconductor at an upper portion of the source and drainelectrodes.

A method for fabricating an oxide thin film transistor, including:forming a gate electrode on a substrate; forming a gate insulating layeron the gate electrode; 1 forming an active layer made of amorphous zincoxide-based semiconductor on the gate insulating layer; forming sourceand drain electrodes electrically connecting with a certain region ofthe active layer on the substrate with the active layer formed thereon;forming a first protection layer formed of silicon nitride film byinjecting only N₂ gas by using a stopper equipment on the substrate withthe source and drain electrodes formed thereon, the first protectionlayer having a thickness of 100 Å to 400 Å; and forming a secondprotection layer on the first protection layer.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of embodiments as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIG. 1 is an exploded perspective view schematically showing a generalliquid crystal display (LCD) device;

FIG. 2 is a sectional view schematically showing the structure of ageneral oxide thin film transistor;

FIG. 3 is a sectional view schematically showing the structure of anoxide thin film transistor (TFT) according to a first embodiment;

FIG. 4 is a sectional view schematically showing the structure of anoxide thin film transistor (TFT) according to a second embodiment;

FIGS. 5A to 5C are sectional views sequentially showing a fabricationprocess of the oxide TFT of FIG. 4;

FIG. 6 is a graph of transfer characteristics of the oxide TFT accordingto the second embodiment;

FIG. 7 is a sectional view schematically showing the structure of anoxide thin film transistor (TFT) according to a third embodiment; and

FIGS. 8A to 8E are sectional views sequentially showing a fabricationprocess of the oxide TFT of FIG. 7.

DETAILED DESCRIPTION OF THE DRAWINGS AND THE PRESENTLY PREFERREDEMBODIMENTS

An oxide thin film transistor (TFT) according to exemplary embodimentsof the present disclosure will now be described in detail with referenceto the accompanying drawings.

FIG. 3 is a sectional view schematically showing the structure of anoxide thin film transistor (TFT) according to a first embodiment of thepresent disclosure. Specifically, FIG. 3 shows the structure an oxideTFT using amorphous zinc oxide-based semiconductor as an active layer.

As illustrated, the oxide TFT according to a first embodiment of thepresent disclosure includes a gate electrode 121 formed on a certainsubstrate 110, a gate insulating layer 115 formed on the gate electrode121, source and drain electrodes 122 and 123 formed on the gateinsulating layer 115, and an active layer 124 made of amorphous zincoxide-based semiconductor and electrically connected with the source anddrain electrodes 122 and 123.

In the oxide TFT according to the first embodiment of the presentdisclosure, because the active layer 124 is formed by using amorphouszinc oxide-based semiconductor, high mobility and constant current testconditions are met, and uniform characteristics are secured, so theoxide TFT can be applicable to a large-scale display.

The zinc oxide (ZnO) is a material that can implement three qualities ofconductivity, semiconductor characteristics and resistance according tocontent of oxygen, so the oxide TFT employing the amorphous zincoxide-based semiconductor material as the active layer 124 can beapplicable to a large-scale display including an LCD device and anorganic electroluminescence display.

Recently, enormous interests and activities are concentrated on atransparent electronic circuit, and the oxide TFT employing theamorphous zinc oxide-based semiconductor material as the active layer124 has the high mobility and can be fabricated at a low temperature, soit can be employed for the transparent electronic circuit.

In particular, in the oxide TFT according to the first embodiment of thepresent invention, the active layer 124 is formed with a-IGZOsemiconductor containing heavy metal such as indium (In) and gallium(Ga) in ZnO.

The a-IGZO semiconductor is transparent, allowing visible ray totransmit therethrough, and the oxide TFT fabricated with the a-IGZOsemiconductor has mobility of 1˜100 cm²Ns, exhibiting high mobilitycharacteristics compared with the amorphous silicon TFT.

In addition, the a-IGZO semiconductor has wide band gap and can be usedfor fabricating a UV light emitting diode (LED) having high colorpurity, a white LED and other components. Also, because it can beprocessed at a low temperature, a light and flexible product can bemanufactured.

Moreover, the oxide TFT fabricated with the a-IGZO semiconductor hasuniform characteristics similar to that of the amorphous silicon TFT, soits component structure is simple like the amorphous silicon TFT andthus, the oxide TFT can be applicable to a large-scale display.

In the oxide TFT having such characteristics according to the firstembodiment of the present invention, a carrier density of the activelayer 124 can be adjusted by adjusting an oxygen density in a reactivegas during sputtering, to thereby adjust device characteristics of theTFT.

In addition, in the oxide TFT according to the first embodiment of thepresent invention, after the source and drain electrodes 122 and 123 areformed, the a-IGZO oxide semiconductor is deposited to form the activelayer 124, thus basically solving a degeneration problem of the oxidesemiconductor generated in etching the source and drain electrodes asmentioned above.

Also, because an etching process of the source and drain electrodes canbe freely applied without any limitation, the source and drainelectrodes can be formed as a dual-layer to improve ohmic-contactcharacteristics between the oxide semiconductor and the source and drainelectrodes. This will now be described in detail in a second embodimentof the present invention.

FIG. 4 is a sectional view schematically showing the structure of anoxide thin film transistor (TFT) according to a second embodiment of thepresent invention. The oxide TFT according to the second embodiment ofthe present invention includes the same elements as those of the oxideTFT of the first embodiment of the present invention, except that thesource and drain electrodes are configured as a dual-layer.

As shown in FIG. 4, the oxide TFT according to a second embodiment ofthe present disclosure includes a gate electrode 221 formed on a certainsubstrate 210, a gate insulating layer 215 formed on the gate electrode221, source and drain electrodes 222 and 223 formed on the gateinsulating layer 215, and an active layer 224 made of amorphous zincoxide-based semiconductor and electrically connected with the source anddrain electrodes 222 and 223.

In the oxide TFT according to the second embodiment of the presentdisclosure, because the active layer 224 is formed by using amorphouszinc oxide-based semiconductor like the oxide TFT according to the firstembodiment of the present invention, high mobility and constant currenttest conditions are met, and uniform characteristics are secured, so theoxide TFT can be applicable to a large-scale display.

In particular, in the oxide TFT according to the second embodiment ofthe present invention, the active layer 224 is formed with a-IGZOsemiconductor containing heavy metal such as indium (In) and gallium(Ga) in ZnO.

In the oxide TFT having such characteristics according to the secondembodiment of the present invention, a carrier density of the activelayer 224 can be adjusted by adjusting an oxygen density in a reactivegas during sputtering, to thereby adjust device characteristics of theTFT.

In addition, in the oxide TFT according to the second embodiment of thepresent invention, after the source and drain electrodes 222 and 223 areformed, the a-IGZO oxide semiconductor is deposited to form the activelayer 224, thus basically solving a degeneration problem of the oxidesemiconductor generated in etching the source and drain electrodes asmentioned above.

In particular, the source and drain electrodes 222 and 223 are formed asa dual-layer to improve ohmic-contact characteristics between the oxidesemiconductor, namely, the active layer 224, and the source and drainelectrodes. The source and drain electrodes 222 and 223 includes firstsource and drain electrodes 222 a and 223 a contacting with the gateinsulating layer 215 and second source and drain electrodes 222 b and223 b formed on the first source and drain electrodes 222 a and 223 aand contacting with the active layer 224.

The second source and drain electrodes 222 b and 223 b directlycontacting with the active layer 224 may be made of metal such astitanium (Ti) or a Ti alloy having good bonding force with oxygen orindium tin oxide (ITO), molybdenum, and the like, having goodohmic-contact characteristics with the a-IGZO oxide semiconductor. Thiswill now be described in detail through a method for fabricating anoxide TFT as follows.

FIGS. 5A to 5C are sectional views sequentially showing a fabricationprocess of the oxide TFT of FIG. 4.

As shown in FIG. 5A, the gate electrode 221 is formed on the substrate210 made of a transparent insulating material.

In this case, amorphous zinc oxide-based composite semiconductor appliedto the oxide TFT according to the present disclosure can be deposited ata low temperature, so it can be used for a plastic substrate or thesubstrate 210 that can be applicable to a low temperature process suchas soda lime glass and the like. In addition, because the amorphous zincoxide-based composite semiconductor has amorphous characteristics, itcan be used for the large-scale display substrate 210.

The gate electrode 221 is formed by depositing a first conductive filmon the entire surface of the substrate 210 and selectively patterning itvia a photolithography process (first masking process).

Here, the first conductive film can be made of a low-resistance opaqueconductive material such as aluminum (Al), an aluminum alloy, tungsten(W), copper (Cu), nickel (Ni), chromium (Cr), molybdenum (Mo), titanium(Ti), platinum (Pt), tantalum (Ta), or the like. Also, the firstconductive film may be made of an opaque such as ITO, indium zinc oxide(IZO), and the like, or may be formed with a multi-layered structure bystacking two or more conductive materials.

As shown in FIG. 5B, an inorganic insulating layer such as a siliconnitride film (SiNx), silicon oxide film (SiO₂) and the like is formed orthe gate insulating layer 215 made of high dielectric oxide film such ashafnium (Hf) oxide or aluminum oxide is formed on the entire surface ofthe substrate 210 with the gate electrode 221 formed thereon.

The gate insulating layer 215 may be formed through a chemical vapordeposition (CVD) or a plasma enhanced chemical vapor deposition (PECVD).

The second conductive film may be used regardless of types of metals toform the source and drain electrodes 222 a and 223 a on the gateinsulating layer 215, and the third conductive layer may be made oftitanium or titanium alloy having good bonding force with oxygen ormetal such as ITO, molybdenum, or the like, having good ohmic-contactcharacteristics with the a-IGZO oxide semiconductor. The source anddrain electrodes 222 and 223 may be formed to have a multi-layeredstructure of dual or more layers.

With reference to FIG. 5C, amorphous zinc oxide-based semiconductor isdeposited on the entire surface of the substrate 210 with the source anddrain electrodes 222 as a dual-layer formed thereon to form a certainamorphous zinc oxide-based semiconductor layer, which is thenselectively patterned through a photolithography process (third maskingprocess) to form the active layer 224 electrically connecting with thesecond source and drain electrodes 222 b and 223 b.

In this case, the amorphous zinc oxide-based composite semiconductor,particularly, the a-IGZO semiconductor may be formed according tosputtering by using a composite target of gallium oxide (Ga₂O₃), indiumoxide (In₂O₃) and zinc oxide (ZnO), and besides, it can be formed byusing the CVD, atomic layer deposition (ALD), and the like.

In addition, the amorphous zinc oxide-based semiconductor layer, i.e.,the a-IGZO, may be formed by using a composite oxide target having atomratios of 1:1:1, 2:2:1, 3:2:1, 4:2:1, etc. of gallium, indium, and zinc.

Here, in the oxide TFT according to the present invention, a carrierdensity of the active layer 224 can be adjusted by adjusting an oxygendensity in a reactive gas during sputtering to form the amorphous zincoxide-based semiconductor layer, and at this time, uniform devicecharacteristics can be obtained under conditions that oxygen density isabout 1% to about 20% and the thickness is about 500 Å to about 1000 Å.

FIG. 6 is a graph of transfer characteristics of the oxide TFT accordingto the second embodiment of the present invention, in which transfercharacteristics of a-GIZO semiconductor TFTs with a channel region witha width of 100 μm and a length of 6 μm.

The graph of the transfer characteristics of the oxide TFT according tothe second embodiment of the present invention shows drain currentsaccording to the change (−15V˜30V) in the gate voltage while the drainvoltages are maintained as 0.1V and 10V.

As shown in FIG. 6, it is noted that the oxide TFT according to thesecond embodiment of the present invention has a higher on-current andhigh mobility characteristics with a steep slope compared with theamorphous silicon TFT.

In particular, as for the oxide TFTs according to the first and secondembodiments of the present invention, because the active layer made ofoxide semiconductor is prevented from being damaged when the source anddrain electrodes are formed, the on-current and slope characteristicsare further improved. In addition, the oxide TFT according to the secondembodiment of the present invention includes the source and drainelectrodes formed as a dual-layer, improving the ohmic-contactcharacteristics between the active layer and the source and drainelectrodes. Thus, excellent device characteristics can be obtained.

FIG. 7 is a sectional view schematically showing the structure of anoxide thin film transistor (TFT) according to a third embodiment of thepresent invention, showing an oxide TFT structure in which a siliconnitride film is deposited by using a sputter equipment without H₂ gasand used as a protection layer of the oxide semiconductor.

As shown in FIG. 7, the oxide TFT according to a third embodiment of thepresent invention includes a gate electrode 321 formed on a certainsubstrate 310, a gate insulating layer 315 a formed on the gateelectrode 321, an active layer 324 formed as amorphous zinc oxide-basedsemiconductor on the gate insulating layer 315 a, source and drainelectrodes 322 and 323 electrically connecting with a certain region ofthe active layer 324, and a protection layer 315 b having adual-structure formed on the source and drain electrodes 322 and 323.

The protection layer 315 b of the dual-structure according to the thirdembodiment of the present invention has the dual-structure of a firstprotection layer 315′ formed by depositing only nitrogen (N₂) gas byusing the sputter equipment and a second protection layer 315″ depositedby adding an inert gas such as a small amount of argon (Ar) gas toincrease a deposition speed.

At this time, in the oxide TFT according to the third embodiment of thepresent invention, because the active layer 324 is formed by usingamorphous zinc oxide-based semiconductor like the oxide TFT according tothe first embodiment of the present invention, high mobility andconstant current test conditions are met, and uniform characteristicsare secured, so the oxide TFT can be applicable to a large-scaledisplay.

In particular, in the oxide TFT according to the third embodiment of thepresent disclosure, the active layer 324 is formed with a-IGZOsemiconductor containing heavy metal such as indium (In) and gallium(Ga) in ZnO.

In the oxide TFT having such characteristics according to the thirdembodiment of the present invention, a carrier density of the activelayer 324 can be adjusted by adjusting an oxygen density in a reactivegas during sputtering, to thereby adjust device characteristics of theTFT.

In addition, in the oxide TFT having such characteristics according tothe third embodiment of the present disclosure, in order to solve theproblem of degradation of the oxide semiconductor due to H₂ gas whilethe PECVD SiO₂ is deposited as described above, the protection layer 315b is formed to have the dual-structure including the first protectionlayer 315′ formed by depositing only N₂ gas by using the sputterequipment and the second protection layer 315″ deposited by adding aninert gas such as a small amount of Ar gas to increase a depositionspeed. This will now be described in detail through a method forfabricating an oxide TFT as follows.

FIGS. 8A to 8E are sectional views sequentially showing a fabricationprocess of the oxide TFT of FIG. 7.

As shown in FIG. 8A, the certain gate electrode 321 is formed on thesubstrate 310 made of a transparent insulating material.

The gate electrode 321 is formed by depositing a first conductive filmon the entire surface of the substrate 310 and selectively patterning itvia a photolithography process (first masking process).

Here, the first conductive film can be made of a low-resistance opaqueconductive material such as aluminum (Al), an aluminum alloy, tungsten(W), copper (Cu), nickel (Ni), chromium (Cr), molybdenum (Mo), titanium(Ti), platinum (Pt), tantalum (Ta), or the like. Also, the firstconductive film may be made of an opaque such as ITO, indium zinc oxide(IZO), and the like, or may be formed with a multi-layered structure bystacking two or more conductive materials.

As shown in FIG. 8B, an inorganic insulating layer such as a siliconnitride film (SiNx), silicon oxide film (SiO₂) and the like is formed orthe gate insulating layer 315 a made of high dielectric oxide film suchas hafnium (Hf) oxide or aluminum oxide is formed on the entire surfaceof the substrate 310 with the gate electrode 321 formed thereon.

The amorphous zinc oxide-based semiconductor is deposited on the entiresurface of the substrate 310 with the gate insulating layer 315 a formedthereon to form the certain amorphous zinc oxide-based semiconductorlayer, which is then selectively patterned through a photolithographyprocess (second masking process) to form the active layer 324 made ofthe amorphous zinc oxide-based semiconductor on the gate electrode 321.

In this case, the amorphous zinc oxide-based semiconductor layer, i.e.,the a-IGZO, may be formed by using a composite oxide target having atomratios of 1:1:1, 2:2:1, 3:2:1, 4:2:1, etc. of gallium, indium, and zinc.

Here, in the oxide TFT according to the third embodiment of the presentinvention, a carrier density of the active layer 324 can be adjusted byadjusting an oxygen density in a reactive gas during sputtering to formthe amorphous zinc oxide-based semiconductor layer, and at this time,uniform device characteristics can be obtained under the condition thatoxygen density is 1% to 20%.

As shown in FIG. 8C, the second conductive layer is formed on the entiresurface of the substrate 310 with the active layer 324 formed thereonand selectively patterned through a photolithography process (thirdmasking process) to form the source and drain electrodes 322 and 323formed of the second conductive film and electrically connecting withthe source and drain electrodes of the active layer 324 on the activelayer 324.

As shown in FIG. 8D, the first protection layer 315′ formed of thesilicon nitride film is formed with a thickness of about 100 Å to about1000 Å (preferably, about 100 Å to about 400 Å) by using the sputterequipment.

In this case, the first protection layer 315′ positioned on the activelayer 324 serves to protect a back channel of the active layer 324, andby depositing it by injecting only N₂ gas during sputtering by usingsilicon target, degradation of the oxide semiconductor due to H₂ or Argas can be prevented during a process.

At this time, in order to improve uniformity of the deposited firstprotection layer 315′, a nozzle does not jet N₂ gas directly to cathodebut jets it to a wall surface of the sputter chamber so as to bereflected and introduced to the cathode.

Thereafter, as shown in FIG. 8E, during the sputtering, an inert gassuch as Ar gas is injected additionally to form the second protectionlayer 315″ formed of the silicon nitride film on the first protectionlayer 315′ at a low power level. In this embodiment, sputtering isperformed at a low power level to form the second protection layer 315″,but the present invention is not limited thereto and the sputtering maybe performed at a high power level to increase the deposition speed.

For the second protection layer 315″, the inert gas such as N₂ gas andAr gas are used, so the deposition speed of the second protection layer315″ is faster than that of the first protection layer 315′, and besidesthe silicon nitride film using sputtering, the second protection layer315″ may be formed of an organic insulating layer or a high dielectricoxide film such as hafnium oxide or aluminum oxide.

In addition, another inorganic insulating layer of organic insulatinglayer may be formed on the second protection layer 315″ to make theprotection layer 315 b have two or more types of dielectric constantsand refractive indexes.

As described above, the present invention may be applied for a displaydevice fabricated by using TFTs as well as the LCD device, for example,an organic electroluminescent display device in which an organicelectroluminescent element is connected to a driving transistor.

In addition, because the amorphous zinc oxide-based semiconductormaterial that has a high mobility and can be processed at a lowtemperature is applied as the active layer, the present invention can beadvantageously used for a transparent electronic circuit or a flexibledisplay.

As the present invention may be embodied in several forms withoutdeparting from the spirit or essential characteristics thereof, itshould also be understood that the above-described embodiments are notlimited by any of the details of the foregoing description, unlessotherwise specified, but rather should be construed broadly within itsspirit and scope as defined in the appended claims, and therefore allchanges and modifications that fall within the metes and bounds of theclaims, or equivalents of such metes and bounds are therefore intendedto be embraced by the appended claims.

1. A method for fabricating an oxide thin film transistor, the methodcomprising: forming a gate electrode on a substrate; forming a gateinsulating layer on the substrate; forming first source and drainelectrodes on the gate insulating layer; forming second source and drainelectrodes on the first source and drain electrodes, wherein the secondsource and drain electrodes are made of titanium or titanium alloyhaving good bonding force with oxygen; and forming an active layer madeof amorphous zinc oxide-based semiconductor at an upper portion of thesecond source and drain electrodes.
 2. The method of claim 1, whereinthe substrate is formed as glass substrate or a plastic substrate. 3.The method of claim 1, wherein the active layer is made of amorphousindium gallium zinc oxide (a-IGZ0) semiconductor.
 4. The method of claim3, wherein the active layer is formed with a thickness of 500 Å to 1000Å containing an oxygen density of about 1% to about 20% in a reactivegas during sputtering.
 5. The method of claim 4, wherein the a-IGZOsemiconductor is formed by using a composite oxide target having atomratios of 1:1:1, 2:2:1, 3:2:1, 4:2:1 of gallium, indium, and zinc.
 6. Anoxide thin film transistor (TFT) comprising: a gate electrode on asubstrate; a gate insulating layer on the gate electrode; first sourceand drain electrodes on the gate insulating layer; second source anddrain electrodes on the first source and drain electrodes, wherein thesecond source and drain electrodes are made of titanium or titaniumalloy having good bonding force with oxygen; and an active layer on thesecond source and drain electrodes comprised of amorphous zincoxide-based semiconductor.
 7. The TFT of claim 6, wherein the substrateis formed as a glass substrate or a plastic substrate.
 8. The TFT ofclaim 6, wherein the active layer comprises amorphous indium galliumzinc oxide (a-IGZ0) semiconductor.
 9. The TFT of claim 6, wherein theactive layer has a thickness of about 500 Å to about 1000 Å containingan oxygen density of about 1% to about 20% in a reactive gas duringsputtering.